In the field of drug delivery systems, liposome formulations containing drugs such as anticancer drugs have been widely studied in the past. In such studies, various approaches to modify liposome surfaces with water-soluble polymers have been made for the purpose of increasing the retention of the liposome formulations in blood. Since a surface modified with a water-soluble polymer, for example, polyethylene glycol, has a hydration layer formed therearound, a liposome formulation thereof is unlikely to be incorporated into reticuloendothelial system (RES) tissues in blood and therefore is increased in retention in blood. In general, blood vessels around cancer tissues remain in a state of high permeability. Therefore, the longer the residence time of particles like liposomes in blood is, the more the opportunities to be incorporated into the cancer tissues are, which allows the integration efficiency of formulations to be increased.
A polyethylene glycol-modified lipid (PEG lipid) bound to a hydrophobic compound is widely used as a polyethylene glycol derivative used to modify liposomes. A polyethylene glycol-modified phospholipid (PEG phospholipid) such as diacyl phosphatidylethanolamine is cited as an example thereof. However, in recent years, the following disadvantages have been pointed out: a disadvantage that the amount of a phospholipid incorporated into cells cannot be earned because the charge of the phospholipid repels the membranes of the cells, a disadvantage that a drug cannot be efficiently contained in a liposome depending on the charge of the drug, and the like. Therefore, a diacyl glycerol-type PEG lipid having no charge is believed to be a more useful material depending on liposome formulations.
On the other hand, attempts to actively increase the delivery efficiency of formulations are being made in such a way that molecules, such as antibodies, peptides, sugar chains, and ligands, recognizing target sites are grafted to PEG terminuses. In order to graft such molecules to the PEG terminuses, the PEG terminuses need to be converted into reactive active groups in advance. For the active groups, the following groups are selected: functional groups suitable for reactions with reactive sites (for example, an amino group of a lysine side chain and a thiol group of a cysteine side chain) of antibodies, peptides, sugar chains, ligands, and the like. When PEG is bonded to an amino group of a lysine side chain or the like, an undesirable situation such as the deactivation of an active site arises in some cases. In general, cysteine is less in amount than lysine among amino acids contained in biologically relevant molecules such as proteins; hence, there is an advantage that the bonding of PEG to thiol groups is unlikely to cause the deactivation of active sites. Against such a background, for phospholipids conventionally widely used, thiol-reactive PEG phospholipids have been developed; however, only a small number of diacyl glycerol-type thiol-reactive PEG lipids have been developed.
While any example of a thiol-reactive PEG lipid is not disclosed, Patent Literature 1 describes a diacyl glycerol-type activated PEG lipid and is cited as one synthesis example applicable to the synthesis of a thiol-reactive PEG lipid. In a production method described therein, a PEG lipid protected with a benzyl ether group is deprotected under reducing conditions to produce a hydroxyl group and the hydroxyl group is converted into p-nitrophenyl carbonate, whereby an activated PEG lipid is obtained. However, since the PEG lipid is exposed to such reducing conditions, an ester group that is a linking group of the lipid is likely to be decomposed and the co-production of a monoacyl lipid is likely to be caused. Since relatively severe conditions such as oxidative conditions, acidic conditions, and alkaline conditions are required for the deprotection of a common protective group other than the exemplified benzyl ether group, the co-production of the monoacyl lipid cannot be avoided.
Furthermore, Patent Literature 2 is cited as a production method excluding deprotection. In the described production method, after the reaction of amino groups of D-glucamine with activated polyethylene glycol chains having maleimide groups, lipid chains are grafted to hydroxyl groups of D-glucamine. However, in this method, the lipid chains are not grafted to all the hydroxyl groups of D-glucamine because of steric hindrance and the like and therefore the co-production of a monoacyl lipid is caused. This problem is not mentioned therein.
Co-produced monoacyl lipids are generally referred to as lysolipids. It has been reported that the lysolipids destabilize liposomes, have strong biotoxicity, and exhibit bioactivity. These problems are highly valued in the case of using lipids as drug delivery systems and therefore a high-purity thiol-reactive polyoxyalkylene-modified lipid having a small lysolipid content is desired.